Low-temperature storage of transplants at the light compensation point: air temperature and light intensity for growth suppression and quality preservation

A method for storing transplants in vitro was developed using light compensation points in conjunction with low temperatures. Broccoli (Brassica oleracea L., cultivar 'Ryokurei') plantlets, aseptically germinated and cultured for 3 weeks in vitro, were used as model transplants. Culture conditions were: 23 °C air temperature, 160 μmol m^-2 s^-1 photosynthetic photon flux density (PPFD), and 3.6 air exchanges per hour of the vessel. Prior to storage, CO₂ exchange rates of the plantlets were measured at 3, 5, 10, 15 and 25 °C air temperatures under 0 (darkness), 2, and 4 μmol m^-2 s^-1 PPFD to determine light compensation points of the plantlets cultured with or without 20 g l^-1 sucrose in the medium. Plantlets were stored for 6 weeks at 5, 10, and 15 °C under either 0 or 2 μmol m^-2 s^-1 continuous PPFD, which was near their light compensation points at these temperatures. Results from the measurements of CO₂ exchange rates showed that the light compensation points varied with air temperature and with medium sugar level. Plantlet dry weight during storage was best maintained by keeping CO₂ exchange rate of the plantlets close to zero throughout the storage period. High transplant qualities were successfully preserved at light compensation points: 2 μmol m^-2 s^-1 PPFD at 5-10 °C without sugar, and at 5 °C with sugar in the medium. This method may also be applicable for storage of other crop transplants, plug seedlings and cuttings.